Activity counting device in multichannel arrangement



Jan. 21, 1969 l. PELAH ETAL 3,423,586

ACTIVITY COUNTING DEVICE IN MULTICHANNEL ABRANGEMENT Filed Nov. 27, 1964Sheet of e .22D m ZOTFOmmE A TTORNE YS ACTIVITY COUNTING DEVICE INMULTICHANNEL ARRANGEMENT Sheet Filed Nov. 27, 1964 lllllllllllllllllllllIIJ 1\, 2

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ATTORNEYS Jan. 21, 1969 3,423,586

ACTIVITY COUNTING DEVICE 1N MULTICHANNEL ARRANGEMENT l. PELAH ET Al-Sheet Filed Nov. 27, 1964 INVENTORS Georges FRAYSSE Walter HAGE IsraelFELAH ATTORNEYS Sheet 5 of Jan. 21, 1959 l, PELAH ETAL ACTIVITY COUNTINGDEVICE IN MULTICHANNEL ARRANGEMBNT Filed NOV. 27, 1964 A TTORNE YS Jan.21, 1969 pELAH ETAL ACTIVITY COUNTING DEVICE IN MULTICHANNEL ARRANGEMENTFiled Nov. 27, 1964 Sheet 5:29. t 5:2.; Il o: 2 2 t 2 m. Z n. N. 2

INVENTORS Georges FRAYSSE walter HAGE Israel PELAH A TTORNE YS UnitedStates Patent O 50,057/ 63 U.S. Cl. Z50-83.1 2 Claims Int. Cl. G01t3/00; H013 39/32 ABSTRACT OF THE DISCLOSURE There is provided a foilactivity counting device for measuring a neutron ux emitted fromradioactive materials. The device comprises a rotating sample carrierunit having samples of radioactive materials deposited thereon, aradiation detection unit mounted in a stationary fashion on the carrierunit, a single measuring channel connected to the radiation detectionunit wlhich analyses and amplifies the output signals of the radiationdetection unit, a pulse emitter controlled by the rotating samplecarrier unit, a single addressing channel connected to the pulse emitterand providing identification pulses in synchronism with the carrierrotation, a multichannel pulse height analyser connected to the outputsrespectively of the addressing channel and the measuring channel inorder to receive distribution of the pulses from the radiation detectionunit. The rotation speed of the rotating sample carrier unit is fixed atsuch a rate that the 'counting time for one foil is negligible ascompared to the drifting time in the measuring channel and the half-timedecay time of the foil substance.

The present invention relates to an activity counting device inmultichannel arrangement as it is used in connection with neutron fluxmeasurements in a nuclear reactor or in similar nuclear assemblies.

Working with foil-samples a series of foils is temporarily located atdifferent points in the reactor, as well horizontally as vertically, andis activated by the neutrons according to the prevailing neutron fluxdistribution. The foils exposed are then taken out of the reactor forcounting the activity in an appropriate counting device in order todetermine by later calculation the neutron flux distribution. Instead offoils, wires or ribbons are in use.

Counting devices for this kind of experiments cornprise necessarilyradiation detectors, appropriate counting channels and an addressableOutput register with data recorder.

When the samples are counted simultaneously by a corresponding number ofdetectors, then an equal number of counting channels is generallyadopted in order to compensate drifting. It is obvious, however, that insuch a multichannel system the number of electronic units for detection,analysing and counting is rather elevated. The other advantage ofsimultaneous counting, the favorable ratio of effective counting time tothe overall operation time for one foil, is partly outweighted by thenecessity of submitting results to half-life period corrections.

It is an object of the present invention, to provide a foil activitycounting device in which drifting is negligible and half-life periodcorrections unnecessary, and yet the number of detention and electronicunits strongly reduced.

Another object of the invention is to realize the electronic concept ofthe counting device in a Way that it accepts as `well as multichannelpulse height analyser, a

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multiscaler, as a multiparameter analyser as addressable outputregister.

Another object of the invention is to arrange and operate the countingdevice in a manner to perform the Fourier analysis of neutron fluxdistributions.

Still another object of the invention is to operate the counting devicein a way to output values for average spatial neutron fluxdetermination.

Still a further object of the invention is the counting device alltransistorized, thus needinlg no heating or cooling.

Another object of the invention is to utilize within the counting devicea foil carrier mechanism which permits continuous `charging anddecharging of the foils and continuous measuring.

The new counting device in its largest scope is characterized accordingto the present invention in that it comprises a high speed travellingsample carrier unit with speed control, a radiation detection unitmounted on the carrier unit, a single -measuring channel connected tothe radiation detection unit which analyses and amplies the outputsignals of the radiation detection unit, an internal synchronizing clockdevice controlled by the travelling sample carrier mechanism, a singleaddressing channel connected to the clock device and a multichannelanalyser as registering unit connected to the outputs of the measuringand addressing channel respectively.

In the addressing channel a continuous series of pulses is generated toassure in the multichannel analyser correct correspondance of thedetector pulses fed in the measuring channel, to the samples originatingthese pulses. In the measuring channel there is provided only a singlechannel analyser and an univibrator as electronic units.

In the case of a multiscaler as registering unit, the pulse series inthe addressing channel is fed immediately to the multiscaler. No furtherelectronic units are involved, unless the multiscaler does not resetautomatically to channel 1. In that case, it is necessary to provide areset pulse at each cycle.

However, when a multichannel pulse height analyser is used asregister-inlg output of the counting device, the essential electronicunits in the addressing channel are a saw tooth carrier generator and astaircase modulator. Both, the addressing channel and the measuring`channel then are connected to a common output unit, for example, to asignal adder unit, followed by a carrier suppression unit. The latterone is connected to the input of the pulse height analyser. The gate ofthat analyser is branched to the measuring channel output.

A travelling foil carrier mechanism should be preferably a rotatabledisk carrying foils on his upper side `and driven by an electric motor.In the example of a counting device with special modulation of theaddressing pulses, a synchronous motor of constant rotation speed of say1500 t./m. for 50 c./s.-net frequency would incite the synchronizingpulse clock device to generate pulses of a frequency of 25 per sec. inthe 'addressing channel. With l0 foils to be measured on the disk-all incircuim.- ferential distribution-and only one radiation detector formeasuring the radiation counts, one addressing pulse occurs for one fullcounting cycle covering l0 foils.

In the case of a multiscaler as registering unit, the pulse frequency of25 c./sec. is multiplied by the number of the foils on the disk in orderto get one pulse for one foil. For the example chosen, the frequencywill become 250 c./sec. Simultaneously in a by-pass the pulses of 25c./sec. are amplified and stretched to obtain reset pulses. In aconsecutive mixer the reset pulses are mixed to the addressing pulsesand fed to the multiscaler. In the case of a multichannel pulse heightanalyser as registering unit, the addressing pulses are transformed bythe saw tooth generator to a correspondingly shaped pulse series of thesame frequency. In order to get correspondance between each foilmeasured and each group of detected radiation counts-each sawtooth-representing one counting cycleis modulated by a staircasemodulator in a way that the step frequency corresponds to the number offoils measured within that cycle, namely 10. Thus for the adoptedexample with 10 foils on the disk the step width is 4 msec.

In the adder unit (or in the amplifier) measured radiationpulses-positive pulses of constant amplitude are added to (or multipliedwith) the said staircase modulated voltage. As there are free distancesbetween the foils, only the first half of each step is utilized for theimpression of the radiation pulses.

Thus, originally constant amplitude of the pulses is augmented accordingto the rising amplitude of each step of the staircase voltage. In thisway reidentication of the measured pulses and its originating foils isprepared.

The invention will now be described broader in detail. Other novel anduseful characterstics and features of the invention will be apparentfrom the description and from the drawings referred to.

FIG. 1 shows the principal electronic diagram of the counting device;

FIG. 2 shows a section of the addressing channel, when a multiscaler asanalyser is used;

FIG. 3 shows a block diagram of a counting device, when a multichannelpulse height analyser as registering unit is applied;

FIGS. 4a to 4d show the electric signals in the circuitry of the deviceof FIG. 3 at the points a, b, c and d;

FIG. 5 shows the mechanical parts of clock device involved in thegeneration of synchronising pulses;

FIG. 6 shows a modified section of the counting device of FIG. 3;

FIG. 7 shows a counting program for Fourier analysis;

FIG. 8 shows a modified version of the foil carrier for variable speed.

According to FIG. 1 the new counting device comprises a high speedtravelling foil carrier unit 1 with speed control 2, a radiationdetection unit 3 mounted on the carrier unit, a single measuring channel4 with appropriate electronic units 5 connected to the radiationdetection unit 3, an internal synchronising clock device 6 controlled bythe travelling foil carrier mechanism 1, a single addressing channel 7with appropriate electronic units 8 connected to the clock device 6, anda multichannel analyser 9 as registering unit connected to the outputsof the measuring and addressing channel respectively.

The high speed travelling foil carrier unit 1 comprises a motor drivenmechanical support on which are xed a number of activated foils orsamples to be measured one after the other, as is described later.

The speed of the support can be controlled by different modes, e.g.constant speed, stepwise advance, variable speed. It is of such a rate,that the counting time for one foil is negligible to the drifting timein the measuring channel or to the half-life decay time of the foilsubstance.

Preferably only one radiation detector is used for the measuring of allfoils. Thus the foils are counted successively. Comparing this countingprocess with multichannel counting, the overall counting time for thesamples is smaller. However, this disadvantage is more than outweightedby the enormous reduction of electronic units and by the fact thatstandard units can be used for all parts of the device.

The internal synchronizing clock of the counting device is a pulseemitter controlled by a moved organ of the travelling carrier device,for instance, by the foil support itself or by a rotating motor part(see further below); in this way synchronizing is assured by a directmechanical link between the foils and the pulse inciting organ.

Especially in the case when the registering unit 9 is a multiscaler,pulses in the addressing channel are immediately fed into the scalerafter appropriate shaping. Charging and clearing of the Scaler memoriesis carried out automatically. As is shown in FIG. 2, the addressingchannel part between the shaper and the multiscaler comprises afrequency multiplier 10, an ampliiier 11 and a mixer 12. Multiplier andamplifier are fed with rectangular addressing pulses of, 'for instance,25 c/sec., 1 cycle representing one counting cycle. In the multiplier,frequency is multiplied by a factor (of 10) according to the number ofsamples counted. In the amplifier 25 c./sec.pulses are amplified inorder to get reset pulses for the multiscaler at the end of eachcounting cycle.

In FIG. 3, a counting device for pulse height analyser application isshown, where the measuring and counting channel have a common output andmodulation of the addressing pulses is provided.

The foil carrier unit is represented by the rotating disk 13, thesynchronous motor 14 and the radiation protection shield 15. The disk isdirectly coupled to the rotorshaft 16 of the motor and capsuled by theshield of lead 17. The shield is provided with a sluice (not shown) tointroduce and to retire samples. Motor windings are connected to aregulated A.C. voltage sources of 50 c./s. over the lines 18.

Mounted on the shield of the foil carrier unit is a single radiationdetection unit, composed of a scintillation counter 19 and aphotornultiplier 20 of the type 56 AVP. The detection unit is located ina zone covering the periphery of the rotating disk, where the samples 20are disposed circumferentially. The measuring channel of the devicecomprises the high voltage supply 21, the negative pole of which isconnected to the cathode of the photomultiplier. The anode is circuitedto earth over a ohm resistance and simultaneously branched to a coaxialtransmission cable 22 of 100 ohms. The cable feeds a tunnel diode singlechannel analyser 23. The analyser is connected with the univibrator 24,which outputs well shaped radiation count pulses of 2 volts amplitudeand 1 lnsec. width.

At point 25 the measuring channel output is branched to a variable delayline 26 and to the coincidence bus of the multichannel pulse heightanalyser 27. The delay time is variable between zero and 500 nsec. Thedead time in the measuring channel must be higher than the dead time ofthe highest channel of the analyser.

The addressing channel of the new counting device commences with aninternal synchronizing clock, which is constituted by the D.C. voltagesource 28, the microswitch 29 and the cam 30 on the motor rotor shaft;see also FIG. 5. The electronic units in the addressing channel are thepulse Shaper 31, the sawtooth generator 32 and the staircase modulator33. Now the output of the channel is branched to the voltage adder 33;see input b. Also the delayed output of the measuring channel isbranched to the voltage adder; see input a. As described earlier, thecount pulses of the measuring channel are impressed in the adder on thestep-modulated synchronizing saw tooth pulses of the addressing channel,in order to prepare reidentification of the signals and foils during andby way of pulse height discrimination.

The adder output, see ref. c, is branched to the carrier voltagesuppression unit 34, and this unit is connected to the input bus of themultichannel pulse height analyser 27; see output d. The analysercomprises 100 channels ranging from 0 to 100 volts, and a screen 35 fordirect display of the counts in two dimensional array. The ordinatereference is the number of counts, the abscissa reference the foilnumber. Attached to the pulse height analyser is fast speed recorder 36.

Signals at the points a, b, c and d of the device of FIG. 3 are shown inthe FIGURES 4a to 4d. The time axis in FIG. 4a is subdivided in 2 msec.intervals from which alternatively one is filled with pulses from asample and one is empty. The empty space is due to the necessarily freespaces between the samples on the rotating disk. Pulses in the intervalbetween 0 and 2 ms. are due to sample No. l, pulses in the intervalbetween 4 ms. and 6 ms. are due to sample No. 2, and so on. The pulsesamplitude is 2 volts, the pulse width is l msec. A dead time could beefficiently used to suppress eventual noise in the interval between 2and 4 ms. and following empty parts.

From FIG. 4b can be seen the way in which a tooth is stepmodulated. Ofthe saw tooth width, which is 400 msec., only the first 8 msec. for thefirst two steps are designed. The first steps amplitude is 5 volts, thesecond steps amplitude is 10 volts, and so on. Thus the amplitude of the10th step is 5 0 volts.

FIGURES 4c and 4d need no further explanation than perhaps this that theoverall voltage of each step is 2 volts greater than the step voltage.

It is clear by this, that according to FIG. 4d a radiation pulse seriesof say l2 volts will enter into channel 12 of the pulse height analyser27, and that these pulses are due to sample No. 2.

There are some general remarks to be made with respect to the countingdevice shown in FIG. 3.

The utilized photo multiplier 56 AVP permits direct connecting of thetunnel diode discriminator by a cable of a certain length, if carefullyadapted. Because of its elevated gain, it can be fed by a small voltagein order to reduce gain drift. Should a less powerful multiplier beused, a preamplifier with three transistors, one of which wired asemitter follower, of a gain of l0 to 20 must be interconnected betweenthe multiplier and the single channel analyser. As to the microswitchunit, it can be substituted by a system in which pulse generating isincited by magnetic, electric or optical means. But when a cam or amagnet is used, it should be angularly adjustable on the shaft.

The coincidence measurement at the pulse height analyser 27 is made inorder to eliminate parasitic pulses due to the step modulator or anothernoise source.

It is effected in advance to the pulse reception in the adder unit. Thisis achieved by retarding the signal in the delay line 26.

With only one detection unit no geometric compensation of themeasurement due to unequal positioning of the foils is possible. Thus asecond detector diametrically opposed to the first one will have to beapplied, This detector must be branched in parallel to the first onewith a delay circuit between, corresponding to the linear speed of thefoil from one to the other. By this means also the relative low countingrate of the device can be ameliorated. With ten detectors in parallel,for instance, the counting rate can be approximated to the half of thatof a multi-measuring channel counting device.

At any rate one high voltage source only is necessary for the detectoror detectors. The gain of each photomultiplier can be controlled by apotentiometer influencing the voltage difference between the anode andthe last dynode of the multiplier when the high voltage is fixed to beconstant and equal for all multipliers.

Another possibility of reducing counting time loss is to operate thefoil disk driving motor step wise. In this case, the transit timebetween counting of two successive foils can be 4made extremely small.When in addition ten detectors in parallel are applied as explainedfurther above, then the counting rate reaches nearly the counting rateof a multimeasuring channel device.

As already mentioned above the adder 33 of the circuit arrangement ofFIG. 3 can be substituted by an amplilier of high gain. This is shown inFIG. 6, Where the saw tooth generator, the step modulator, the delayline and the pulse height analyser are identical with the correspondingunits shown in FIG. 3.

A highly interesting feature of the foil activity counting deviceaccording to the present invention is its convenient application inFourier analysis measurements of the neutron flux of a nuclear reactorfor instance. As can be shown by calculation, consecutive measuring withvariable speed of a continuous elongated sample of varying activity, forinstance a wire or a ribbon, or of discrete samples, as is an array ofaligned foils on a rotating disk, primarily activated in a neutron eldalong an axis of determined direction, yields counting values, which canserve imediately for computation of the neutron flux distribution. Inthe case `of the Fourier analyses of the neutron ux in a reactor corefor instance, samples activated along the vertical (or a radial) axis ofsymmetry of the core can be placed one after the other incircumferential order on the rotating disk of the foil carrier unitexplained above, and measured with a speed depending upon the functionof each term of sum of the Fourier integral respectively. Thus, theconstant term and the harmonics of the integral can be obtained bysubmitting the foils or wires or ribbons to alternative measuringprograms providing speed variations according to inverse cosinus orsinus law versions.

Practically, foils exposed in the central zone of the neutron field aremeasured longer than those exposed above, below or beneath. In FIG. 7 acounting program is shown in which the measuring time t1 to tw for tenfoils varies according to a sinus law. The foils are considered to havebeen exposed in the central vertical axis of a nuclear reactor core,thus foils counted during t1, tw were in positions above and below thebulk ux, While foils -counted t5, t6 were in the centre.

There are several practical methods to perform Fourier analysis with thedescribed counting device. When the drive motor of the foil carrier diskis a synchronous motor, two possibilities exist, either a constructionalone or an operational one. In FIG. 8 is shown schematically aconstructional version of carrier device, which differs from that inFIG. 3 in that it is composed by two independent disks 38, 39. Balls 40on the lower disk support the upper disk permitting relative rotationalmovement of this disk with respect to the other. Foils 41 to be countedare placed 'on the upper disk in circumferential array. The drivingshafts 42, 43 of the device are mounted coaxially and are equipped withgear wheels 44, 45. The gear Wheels mate with corresponding Wheels of aspeed transformer 46, driven by the synchronous motor 47. Disk 38 isdriven with constant basis speed entraining disk 39. The speed of disk39 is varied by the speed transformer 46 which adds to the basis speed asupplemental speed amount, modulated according to an inverse sinus orcosinus law. Cams may be involved in this control action.

An operational method without changing the carrier device in FIG. 3would be to vary the rotation speed of the drive motor according to asinus or cosinus law over a greater number of rotations. It has beenfound that speed variation over n rotations yield the same countingresults that a unique rotation with a device like that in FIG. 8 or astep-motor device described hereafter.

Speed variation over a greater number of rotations, e.g. rotations, iseasy to realise with a voltage controlled commutator motor and even witha synchronous motor, as the frequency of it is to be varied only slowly.

Instead olf using a speed transformer with cams for the reproduction ofthe motion function, control can also be effected on the basis of adiagram paper sheet, Whereon the motion function is traced in aconductive line palpated by voltage fed contacts. The motion function isthen transformed by an appropriate transformer to a corresponding speedvariation effective at rotorshaft 39 of the device in FIG. 8.

When instead of foil samples an activated wire (or ribbon) is used, thewire is placed in form of a spire on the rotating disk. Step-wiseoperation of the disk is then equivalent to the case of n discretefoils. When the disk is driven by a synchronous motor within the deviceof FIG. 3, the stair-case modulator is no longer necessary.

The counting program shown in FIG. 7 can also be realized by aid of astepping motor coupled to a sole foil :carrier disk as shown in FIG. 3.In principal each possible motion law can be simulated by the devicesdescribed above.

When average spatial neutron flux determination shall be can'ed out,measured pulses merely are summed up in a multiscaler, for instancebranched in parallel to the pulse height analyser. This operation issimple Vbecause of the serial mode of function of the counting device.

In the foregoing description, application of either a multichannel pulseheight analyzer of a multiscaler as registering unit was explained.There is however also an interest of utilising multidimensionalanalysers. It arises particularly from neutron eld analysis in fastreactors, where apart from the ux distribution also the neutron energyspectrum is interesting. a

Multidimensional or multiparametric display affords time savings in allanalysing work using activated foils, when the spatial distribution `ofa physical quantity shall be established. Possible parameters are thespatial axis xJ y, z of a rectangular three coordinates system, andparameters alpha, beta, gamma, etc. for fixed neutron energy ranges. Ina thermal nuclear reactor, foils of only one specific substance areused, whereas in fast reactors foils of different substances adapted toxed energy ranges, are applied. For instance In 115, Au 197, I 127, Mn55, Cu 63, Lu 176 for thermal neutrons, and P 3l, S 32, Al 27, Si 28 forfast neutrons.

From the foregoing it is clear, that in a counting system withmultiparametric analysing, identity of the foils is tied to at least tworeferences: -ordonnance number and a parameter. Parameters can beintroduced in the counting system by appropriate coders, attached to theanalyser, whereas visualisation on the screen of the analyser willalways take place in producing the number of counts for one parameteronly. The records outputted contain the codes for all parametersinvolved, thus facilitating later computation `by machines. Withmultiparametric analysers a convenient means is offered to obtain notonly the overall neutron population of a reactor, or the fundamentalterm or the harmonics in the case of Fourier analysis, but also thepopulations split up in energy ranges.

We claim:

1. A foil activity counting device for measuring a neutron ux emittedfrom radioactive materials, which comprises a rotating sample carrierunit having samples of radioactive materials deposited thereon, aradiation detection unit mounted in a stationary fashion over saidcarrier unit, a single measuring channel connected lo said radiationdetection unit which analyses and ampliiies the output signals 'of saidradiation detection unit, a pulse emitter controlled by said rotatingsample carrier unit, a single addressing channel connected to said pulseemitter and providing identification pulses in synchronism with thecarrier rotation, a multi-channel pulse height analyser connected to theoutputs respectively of said addressing channel and said measuringchannel to receive distribution of pulses from the radiation detectionunit, the rotation speed of said rotating sample carrier unit beingcarried out at such a rate that the counting time for one foil isnegligible as compared to the drifting time in the measuring channel andthe half-time decay time of the foil substance.

2. A foil activity counting device as defined -in claim 1, wherein theaddressing channel contains a sawtooth generator followed by a staircasemodulator, and that the measuring channel contains a pulse heightanalyser followed by a univibrator, that both channels are connected toa signal adder unit, and that between said adder unit and said analyser,a differentiating element followed by an amplier having controllableamplification factor is connected.

References Cited UNITED STATES PATENTS 2,490,298 12/1949 Ghiorso et al.Z50-106 3,141,977 7/1964 Fratantuno 250-106 RALPH G. WILSON, PrimaryExaminer.

A. B. CROFT, Assistant Examiner.

U.S. Cl. X.R.

